1. Field of the Invention
The present invention relates to a method for manufacturing impregnated cathodes, and more particularly to a method for manufacturing impregnated cathodes, which is capable of simplifying the manufacturing process and reducing the thickness of cathode.
2. Description of the Prior Art
Impregnated cathodes have been manly used in oscilloscopes which require high cathode current. Recently, they have also been applied to electron tubes which are used in televisions, since the electron tubes require a high cathode current, according to the tendency of high resolution and large screen in televisions.
Referring to FIG. 1, there is shown an example of conventional impregnated cathode constructions. As shown in the drawing, the cathode comprises a cathode body 3 including an impregnated pallet 1 radiating electrons and a cylindrical cathode ring 2 opened at upper and lower ends thereof and made of a heat-resistant molybdenum (Mo), the impregnated pallet 1 being fixedly fitted in the cathode ring 2. The impregnated pallet 1 is made by impregnating an electron radiating material being of composite oxides such as BaO, CaO, or Al.sub.2 O.sub.3 into a porous pallet of a heat-resistant metal such as tungsten (W). The cathode also comprises a cylindrical cathode sleeve 4 made of a heat-resistant molybdenmum (Mo) and having a closed upper end. The cathode sleeve 4 is mounted to the cathode body 3 such that the closed upper surface is in close contact with the lower surface of the cathode body 3. Within the cathode sleeve 4, a heater 5 adapted to heat the cathode is disposed.
Now, a conventional method of making the cathode body 3 will be described, in conjunction with FIG. 2.
As shown in FIG. 2, a porous pallet 6 having a plurality of pores is formed by sintering powder of a heat-resistant metal such as tungsten at a high temperature in a reducing atmosphere and then shaping the sintered product into a pallet. Thereafter, an electron radiating material is impregnated into the pallet 6. The impregnation of the electron radiating material is accomplished by heating the electron radiating material at a high temperature in a vacuum or inert gas atmosphere to melt it, and then impregnating the melt into pores of the pallet 6 in the same atmosphere to produce the impregnated pallet 1. The impregnated pallet 1 is then fixedly fitted in the cylindrical ring 2 which was obtained by deep drawing a piece of a heat-resistant metal such as molybdenum.
The fixing between the impregnated pallet 1 and the cathode ring 2 can be accomplished by using a brazing method or a laser welding method both of which metal powder to be melted is used to be filled between the impregnated pallet 1 and the cathode ring 2.
The cathode body 3 having the above-mentioned construction is then attached to the cathode sleeve 4, to form a impregnated cathode construction shown in FIG. 1.
In the above-mentioned method, however, the thickness of the cathode is unnecessarily increased over a desired thickness, since the fixing between the impregnated pallet 1 and the cathode ring 2 is achieved by using a brazing method or a laser welding method. As a result, the cathode body 3 is undesirably thicken, thereby resulting in the lengthening of the time taken for transmitting a heat from the heater 5 to the upper surface of the cathode body 3. Due to the lengthened heat transmission time, the video producing time increases, thereby causing the performance of the impregnated cathode to be deteriorated. Also, the manufacturing process becomes complex, thereby causing the productivity to decrease. Generally, the video producing time means the period of from the turning-on time point of the heater 5 to the time point that a video appears on an electron tube screen, as an electron is radiated from the impregnated pallet 2.